1. Field of the Invention
The present invention relates to serial data communication systems and, more specifically, to channel equalization.
2. Description of the Related Art
As VLSI component geometries continue to shrink, digital speeds on circuit boards and in cables have increased dramatically. At these higher data rates, effects such as loss and loss-induced inter-symbol interference begin to dominate transmission channel degradation factors.
The current standard data rate for backplane interconnects is 2.5 Gb/s. Although, at these frequencies, frequency-dependent loss is not too severe, a pre-emphasis function at the transmitter is sometimes used to compensate for high-frequency loss in the transmission channel.
As communication rates approach 10 Gbps, frequency-dependent loss is a major source of inter-symbol interference. For example, in the common backplane material FR4, the high-frequency loss of a printed wiring board (PWB) trace is approximately 1 dB/inch/10 GHz. For a typical 40-inch interconnect, this represents 40 dB of high-frequency loss.
Transmitter pre-emphasis techniques typically utilize a single-tap or two-tap FIR filter to combat high-frequency loss. These techniques can effectively equalize the channel at lower data rates. However, at higher data rates (e.g., 10 Gbps), the number of taps required in the pre-emphasis filter increases dramatically, which results in high power consumption. Electromagnetic interference (EMI) also becomes an issue as the high-frequency signals are emphasized at the transmitter side. Additionally, to be able to adapt a pre-emphasis filter's transfer function to a particular channel's characteristics, transmit-side pre-emphasis techniques would require a hand-shaking mechanism between a receiver and a transmitter.
Receive-side channel equalization is also used in some systems (e.g., modems). However, these techniques involve the use of high-speed digital circuits employing high-speed, high-resolution (e.g., greater than four bits) analog-to-digital converters. At high data rates (e.g., 10 Gbps), such implementations are impractical, expensive, and consume too much power.